An electronic device for a radio-receiver chain, for example for a user's mobile communication equipment, may include an oscillator unit which produces an oscillator frequency. The oscillator unit may include a digitally-controlled crystal oscillator, which is arranged to produce an oscillator signal having an oscillator frequency. Such an electronic radio-receiver device may also include an automatic frequency control (AFC) module, which is arranged to analog-tune the digitally-controlled crystal oscillator so as to reduce the frequency difference between the oscillator frequency and a carrier frequency of a radio-received signal.
Such a digitally-controlled crystal oscillator has the advantage of having a low cost price, but the oscillator frequency as produced directly by such a digitally-controlled crystal oscillator may exhibit frequency-drift over time. Such drift may be due to temperature variation of the oscillator, in particular when the user equipment is transmitting data uplinks with a high transmission rate. Indeed, such high transmission rates cause a power amplifier of the mobile user equipment to rise in temperature, which in turn causes the temperature of the digitally-controlled crystal oscillator to increase. But, the digitally-controlled crystal oscillator is usually devoid of any temperature variation compensation system. So, the oscillator frequency increases as a consequence of the rise of the oscillator temperature. The amplitude of such frequency increase may be up to 20 Hz (Hertz). Then, received data from high transmission rate radio-received signals are extremely sensitive to any mismatch existing between the oscillator frequency and the carrier frequency. However, the drift of the oscillator frequency which is produced by the digitally-controlled crystal oscillator may also have many causes other than temperature variations.
In addition, the carrier frequency of the radio-received signal may also change in time, depending on the quality of the radio-transmission between the mobile user equipment and a node B from which the radio signal originates. As a consequence of the variations of both the oscillator frequency produced by the oscillator unit and the carrier frequency of the radio-received signal, an apparent drift occurs for due to the difference between these oscillator frequency and carrier frequency. But for obtaining a symbol decoding with a minimum error rate by the radio-receiver chain of the mobile user equipment, it is essential to reduce the difference between the oscillator frequency and the carrier frequency as much as possible.
To this purpose, it is also known that a radio-receiver chain to may additionally comprise a baseband frequency corrector (BFC) module. Such baseband frequency corrector module may be arranged for digitally tuning the oscillator unit, so that the oscillator frequency matches the carrier frequency with residual frequency difference further reduced compared to the efficiency of the automatic frequency control module alone.
Implementation of the HSDPA protocol requires very fine tuning of the oscillator frequency so as to match the carrier frequency, in particular because of the very high downlink transmission rate of the High Speed Dedicated Shared Channel (HS-DSCH). Even a drift amplitude of a few Hertz difference between the oscillator frequency and the carrier frequency has negative impact on the error rate of the symbol decoding.
Also known is that the automatic frequency control module when implemented without a baseband frequency corrector module, or the combination of an automatic frequency control module with the baseband frequency corrector module, operates as a low-pass time-filtering of the frequency difference between the oscillator frequency and the carrier frequency. This time-filtering creates a trade-off between low residual frequency difference and efficiency of the reduction of this frequency difference when drift occurs for the frequency difference with a non-zero drift rate. Put another way, a trade-off occurs between the static performance and the dynamic behaviour for reducing the difference between the oscillator frequency and the carrier frequency.
Then, what is needed is a technique and device that improves the efficiency of controlling the oscillator frequency to better match the carrier frequency, in particular in dynamic conditions.